Week 18

Question 1

Tetrahedral and square planar geometry

Answer 1

Two different types of geometry for the shapes of atoms
E.g when methane is put in both planar and tetrahedral it creates four bonding orbitals in tetrahedral but only one in planar meaning that tetrahedral allows the most electrons to be lowered in energy and also predicts equal bonding between the hydrogens and carbons and all sigma bonds. The square planar has a lone pair so the geometry isn’t as good

Question 2

what can be said for MOs and their symmetry e.g methane

Answer 2

Firstly, each hydrogen is bonding to the carbon using all the carbon s and p orbitals. This is
analogous to the situation of s-p mixing in diatomic molecules, which really just means that
the s and p orbitals are interacting strongly across the bond.3
* Secondly, all of these bonds are identical. (The cartoon is not great at showing why but the
maths proves it)
* Thirdly, there are four full bonding orbitals which adds up to four “single bonds”

we can make the assumption that:
We know that whilst orbitals can mix and result in quite complex orbital diagrams, these will
be dependent upon the symmetry – i.e. all tetrahedral environments will have similar results
and all linear arrangements will have similar results and so on. This is because they all have
the same symmetry. We can therefore just use idealised solutions for the appropriate
symmetries because the shape dictates how orbitals will overlap.
* We can make the assumption that since bonds are almost entirely local, we can ignore all
other atoms and only worry about two atoms at a time.
3
It is a common misconception that s-p mixing means the s and p orbitals on a given atom mix. This is no

Question 3

C-H bond and it’s s-p mixing

Answer 3

C-H bond arises from one s orbital and 3 p orbitals from carbon

Question 4

Hybrid orbitals

Answer 4

Hybrid atomic orbitals arise from examining the best idealised results for s-p mixing to give new orbitals

Hybrid orbitals don’t exist

Question 5

Tetrahedral orbitals

Answer 5

109.5’ is the angle
And the hybrid set of for tetrahedral geometry is sp3 mixing

Question 6

Sp, sp2 and sp3

Answer 6

S means that there is one 2s orbital
P means that there’s one 2p orbital

If there’s a 2 or 3 after the p it means there’s 2 or 3 sets of p orbitals